1. Field of the Invention
The present invention relates to a control system that may be applied to particulate filters that utilize heat treatment as the method of cleaning. The invention includes a method, particularly suitable for cleaning the filter medium of a soot trap for a vehicular exhaust system.
2. Description of the Related Art
Since the mid-1980's, legislation has been increasing in both the United States and Europe to reduce the level of solid emissions from both on- and off-highway diesel powered vehicles. In order to comply with these and other standards such as health and safety and product spoilage issues, exhaust after-treatment specialists have explored a variety of potential soot filter media. The wallflow type ceramic honeycomb filter is the most widely employed filtration technology used in current systems for industrial applications. Wallflow filters provides an answer to the filtration requirement, yet there remains the residual problem of achieving a reliable and repeatable method of cleaning the filter. This residual problem has been the source of extensive engineering research and development as recited in Higuchi, Mochida & Kojima (NGK Insulators Ltd) "Optimized Regeneration Conditions of Ceramic Honeycomb Diesel Particulate Filters", SAE 830078, Feb. 1983; Mizuno, Kitagawa & Hijikata (NGK Insulators Ltd) "Effect of Cell Structure on Regeneration Failure to Ceramic Honeycomb Diesel Particulate Filter", SAE 870010, Feb. 1987 and an SAE paper by Kitagawa, Hijikata, Makino (NGK Insulators Ltd) entitled, "Effects of DPF Volume on Thermal Shock Failures During Regeneration".
Wallflow filter elements are particularly useful to filter particulate matter from diesel engine exhaust gases. Many references disclose the use of wallflow filters which can comprise catalysts on or in the filter to filter and burn off filtered particulate matter. A common ceramic wallflow filter construction is a multi-channel honeycomb structure having the ends of alternate channels on the upstream and downstream sides of the honeycomb structure plugged. This results in a checkerboard-type pattern on either end. Channels plugged on the upstream or inlet end are open on the downstream or outlet end. This permits the gas to enter the open upstream channels, flow through the porous walls and exit through the channels having open downstream ends. The gas pressure forces the gas through the porous structural walls into the channels closed at the upstream end and open at the downstream end. Such structures are primarily disclosed to filter particles out of the exhaust gas stream.
It is desired to remove the particulate matter from the upstream sides of the wallflow filters. One method is to provide a layer of catalyst on the wall to catalyze the ignition of the particulate matter during operation of the filter. Typical patents disclosing such wallflow filter structures include U.S. Pat. Nos. 3,904,551; 4,329,162; 4,340,403; 4,364,760; 4,403,008; 4,519,820; 4,559,193; 4,563,414; 4,411,856; 4,427,728; 4,455,180; 4,557,962; 4,576,774; 4,752,516; 4,510,265, 4,759,892, 5,114,581 and 5,221,484. Other references of interest include U.S. Pat. No. 5,100,632 and Japanese Kokai 3,130,522.
Soot filter with regeneration systems are reviewed in U.S. Pat. Nos. 4,276,066; 4,331,454; 4,319,896 and 4,345,431. U.S. Pat. No. 4,319,896 discloses a control system to clean soot filters. The gas pressure is correspondingly measured and used to help control the system. When back pressure build up reaches a predetermined point, a switch closes. One or more temperature sensors is located in the filter bed. The sensor will close a circuit switch for electrical heater units. Finally, when switches controlled by back pressure and temperatures are closed, a third switch controlled by the fuel pump and the electric heater will be turned on when a lean mixture is being used in the engine. In this way, the burning of the trapped particles is controlled.
A particularly useful particulate emission control filter directed for use for diesel exhaust is presented in "3M Diesel Filters for Particulate Emission Control, Designers Guide" published by 3M Ceramic Materials Department, printed 1994 January and hereby incorporated by reference. There is described a ceramic filter comprising ceramic fiber specified to have 62% Al.sub.2 O.sub.3, 24% SiO.sub.2, and 14% B.sub.2 O.sub.3. The filter specification includes a white continuous fiber having a fiber diameter of 10-12 microns with a fiber density of 2.7 grams per cubic centimeter. The mechanical properties of the fiber include a filament tensile strength of 1.72 GPA, a filament tensile modulus of elasticity of 138 GPA, and elongation of 1.2%. The specified thermal properties are continuous use temperature of 1204.degree. C., short-term use temperature at 1371.degree. C., a lineal shrinkage at 1093.degree. C. of 1.25%, a melting point of 1800.degree. C., a thermal expansion co-efficient (25-500.degree. C.) of 3.0.times.10.sup.-6 .DELTA.L/L.degree. C., and a specific heat of 1046.7 J/Kg..degree.K. The fiber is sold by the 3M Ceramic Materials Department as NEXTEL.TM. FIBER. The above specified properties are for NEXTEL.TM. 312 CERAMIC FIBER.
The NEXTEL.TM. fibers are used to make diesel filters. A 3M diesel filter cartridge is illustrated in FIG. 1. The cartridge is used in a diesel engine exhaust system. Typically, a plurality of filters is assembled within a canister. The number of filter cartridges assembled in a canister is sized to the exhaust flow rates and anticipated regeneration intervals. The ceramic fiber is wound on a resistance heated cylindrical support one end of the heater is open and the opposite end is sealed. The cartridges are assembled to permit exhaust gas to pass into the canister, through the outside of the filter to a hollow inside and exit the canister from the inside of the filter cartridge. Particulate matter is trapped on the closely wound fibers.
During regeneration the electrically heated cylinder can operate at 12 volts DC using 500 watts or more of power. Current to the heater and filter regeneration intervals are determined during system design. During regeneration gas flows into the inside of the filter and exits through the outside of the filter. This gas direction allows the heater to efficiently burn the soot during regeneration, requiring minimal heater power. The electrical resistor heater can be designed to have uniform resistance or variable resistance along the length of the heater and, therefore, the filter. The 3M Diesel Filters Designers Guide discloses systems for a regeneration of the filters where the exhaust flow can be stopped during regeneration. Alternatively, a system is disclosed where one filter bank can be regenerated while a second filter bank can continuously filter exhaust gas.
While a variety of soot filters are known in the art, improvements are desired in the regeneration of systems using such filters.